The present invention is related to systems and methods for implementing source follower circuits. More particularly, the present invention is related to class AB source follower circuits.
Source follower circuits are known in the art, however, known source follower circuits suffer from one or more deficiencies. As an example, one existing source follower circuit 100 is depicted in FIG. 1. Source follower circuit 100 includes four transistors 115, 120, 125, 130 each with a source (labeled ‘s’), a drain (labeled ‘d’) and a gate (labeled ‘g’). The gate of transistor 120 is connected to a voltage input 155; the source of transistor 120 is attached to the drain of transistor 115, the drain of transistor 130, and a resistor 145; and the drain of transistor 120 is connected to a capacitor 135, the gate of transistor 130, and the drain of transistor 125. In addition, the gate of transistor 125 is connected to a resistor 140, which is attached to a first bias voltage 150; and the source of transistor 125 is connected to a power source. The gate of transistor 130 is connected to capacitor 135, the drain of transistor 125, and the drain of transistor 120; and the source of transistor 130 is connected to the power source. The gate of transistor 115 is connected to capacitor 135 and to a resistor 110 that connected to a second bias voltage 160; and the source of transistor 115 is connected to a ground. An output end of resistor 145 is connected to a voltage output 165 at a node 175.
Source follower circuit 100 operates well where you have a capacitive load connected to node 175 or where you have a pure AC input. However, if there is added DC load at node 175, there cannot be a corresponding DC current to the gate of transistor 115 because of capacitor 135, and thus the voltage at the gate of transistor 115 is equal to bias voltage 160. In this case, the DC load at node 175 will be greater than the quiescent current, and source follower circuit 100 is not capable of sinking sustained current greater than the quiescent current I1 as the DC voltage across resistor 145 decays to zero. It should be noted that source follower circuit 100 may be able to provide some sink current, but on an inconsistent basis during transient operation and for a period shorter than the RC time constant defined by resistor 110 and the gate capacitance of transistor 115. Said another way, source follower circuit 100 is not capable of providing a sustained current greater than I1 via resistor 145.
Hence, for at least the aforementioned reasons, there exists a need in the art for advanced circuits capable of a broader range of operation.